<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <ul>
        <li>1</li>
        <li>2</li>
        <li>3</li>
    </ul>
    <script>
      var lis=document.getElementsByTagName("li");
      console.log(lis);
      /* lis.onclick=function(){
          alert(1);
      } */
      for(let i=0;i<lis.length;i++){
          lis[i].onclick=function(){
              alert(i)
          }
      }
    </script>
</body>
</html>